In Wideband Code Division Multiple Access (WCDMA) based systems, high speed data transmission may be enabled by means of the so-called high speed downlink packet access (HSDPA) transmission, which may provide functions such as fast hybrid automatic repeat request (HARQ), adaptive coding and modulation (AMC), and fast cell selection (FCS). A detailed description of these and other function of HSPDA can be found in the third generation partnership project technical report No. 3G TR25.848 release 2000, entitled PHYSICAL LAYER ASPECTS OF UTRA HIGH SPEED DOWNLINK PACKET ACCESS.
At the present it is assumed that with HSDPA, each user equipment receiving data on a high-speed downlink shared channel (HS-DSCH), which is a transport channel, i.e. a channel between the media access control (MAC) layer and the physical (PHY) layer, also allocates and uses an associated dedicated channel (DCH). The dedicated channel may be mapped to a dedicated physical channel (DPCH) in the physical layer. The DPCH is typically divided into a dedicated physical data channel (DPDCH) and a dedicated physical control channel (DPCCH), both in the uplink and in the downlink. Data such as power control commands, transport format information, and dedicated pilot symbols are transmitted on the DPCCH. Information such as diversity feedback information may also be transmitted on DPCCH in the uplink. The HS-DSCH may be mapped to one or several high speed physical downlink shared channels (HS-PDSCH) in the PHY layer.
The associated dedicated channel DCH is usually allocated for both the downlink and the uplink, and is usually used to carry HSDPA-related information and signaling as well as other dedicated data, such as speech and control data. A user terminal may communicate with several base stations (the term base station being used here in some places to indicate what is called a node B in UTRAN (Universal Mobile Telephone System (UMTS) Terrestrial Radio Access Network) specifications) at the same time during what is called soft handover, and in such situations the associated dedicated channel is said to be in soft handover.
In addition to there being a dedicated channel (DCH) associated with an HS-DSCH, there may also be a shared control channel (SCCH) associated with the HS-DSCH. A SCCH can be used to carry HS-DSCH specific information and signaling to users receiving data on the HS-DSCH.
According to current proposals, the dedicated channel (DCH) would be used to inform the user equipment that it has data to be read on the HS-DSCH and SCCH; in such an arrangement, a user would receive an indication on the dedicated channel of data to be read only when there is data for the user. With such use, the dedicated channel would serve as a pointer channel, since it would point the user to the shared channels.
The dedicated channel (DCH) would also convey information about modulation and coding schemes, power levels and similar parameters used for the shared channels. This information could also be sent on the shared channel. If the shared control channel is used carry this information, then it must be transmitted earlier than the corresponding shared is data channel (HS-DSCH). The shared control channel, on the other hand, would be used to carry information that is specific to the data transmitted on the shared data channel, information such as packet numbers for the HARQ process as well as other information. The shared control channel could be sent as a separate code channel (i.e. it could be code multiplexed) or could be sent using the same code channels as the HS-PDSCH (i.e. it could be time multiplexed with the HS-PDSCH).
Unlike the dedicated channel, according to existing proposals, the HS-DSCH is assumed not to be in soft handover; each base station is assumed to have its own shared channel and a user terminal is assumed to receive data on a HS-DSCH from only one base station at a time. The so-called fast cell selection (FCS) technique would be used to switch from one base station to another for receiving data on HS-DSCH. However, the shared channels would not use power control. Instead, the shared channels are proposed to be transmitted either with power that is fixed or with power that is semi-fixed (meaning that the power is not changed too often). The power could, for instance, be a cell-specific parameter.
In the current proposals, the high speed downlink shared channel (HS-DSCH) is planned to be associated with a dedicated channel that would carry in the downlink at least information regarding the timing when the receiving station is to receive on a shared channel. In the uplink, the associated dedicated channel would carry, among other information, required acknowledgements (ACK) used in a so-called fast HARQ, i.e. the HARQ process used by the MAC-high-speed (MAC-hs) layer/entity/service, as explained for explained in 3GPP TR 25.950 v4.0.0 (2001-03) UTRA High Speed Downlink Packet Access.
By way of an explanation of the phraseology used here: a HARQ process is used to indicate what is sometimes called a HARQ ‘channel’; a data block is used here to indicate a HARQ data block and is a block of data transmitted (and retransmitted) by a HARQ entity in MAC-hs. A packet is a general term, and is sometimes used to mean a data block and sometimes a RLC PDU.
Release '99 RLC assumes that packets (RLC-PDUs) are received in order. For unacknowledged mode (UM) service, if an RLC-PDU is missing, the complete RLC service data unit (RLC-SDU) is discarded. For acknowledged mode (AM) service, a missing RLC-PDU causes a retransmission request. If the RLC-PDUs of a message are not received in sequence, some RLC-SDUs may be discarded unnecessarily for UM and some unnecessary RLC-PDU retransmissions may be generated for AM. Therefore, it is advantageous for either MAC-hs to provide in-sequence delivery of the RLC-PDUs of a message or for the RLC layer to be modified to support out of sequence delivery of RLC-PDUs. In addition to data PDUs, there are also RLC-control PDUs, which are not numbered, and so, if received out of sequence, cannot be reordered based on RLC PDU sequence numbers.
If re-sequencing is implemented at the MAC layer using the MAC-hs, HARQ data block numbering by MAC-hs is required. (RLC-PDU numbering is usually not known at the MAC layer.) This data block numbering should be across the HARQ processes (or the N HARQ ‘channels’ as they are called in TR 25.950) to recover from lost TTIs (transmission time intervals), i.e. TTIs for which the user terminal identifier cannot be read. A TTI is the time between consecutive deliveries of data between the medium access control (MAC) layer and the L1 transport layer, and so defines the periodicity at which Transport Block Sets are transferred to the physical layer on the radio interface. HARQ processes are the same as HARQ channels as described in TR 25.950. There are N HARQ processes, each operating with stop-and-wait (SAW) protocol. The incoming data blocks are distributed to different HARQ processes. The receiver has to know which HARQ process is being received at each moment. Therefore, the HARQ process number has to be sent on the shared control channel.
If longer HARQ data block numbers across the HARQ processes are used, HARQ process numbers are not needed since the soft combining of first transmissions and retransmissions of the same block can be based on the HARQ data block number. Using HARQ data block numbers makes the HARQ scheme similar to a selective repeat (SR) scheme. (In order to control the re-sequencing buffer sizes, some transmit and receive windows should be specified.)
Asynchronous HARQ requires that the HARQ process number be signaled in the downlink. If there are N=6 subchannels (i.e. 6 HARQ processes), 3 bits are needed to signal the HARQ process number. In addition, at least one bit sequence number is needed per HARQ process (channel) to recover from errors in ACK/NACK. This implies that at least four-bit ‘sequence numbers’ are needed with asynchronous N-channel HARQ. Four-bit sequence numbers would not, however, guarantee in-sequence delivery of the packets (RLC-PDUs). The SAW protocol guarantees that within each HARQ process the data blocks are delivered in order. However, it is possible that a data block in one HARQ process goes through faster (with less retransmissions) than another (earlier) data block in another HARQ process. Furthermore, if a data block is totally missed in between (i.e. the UE does not know whether the lost block was intended for it or for some other UE), then the UE can not continue to keep track of the correct order of the data blocks.
What is needed is an asynchronous N-channel HARQ scheme (i.e. an N-process HARQ scheme) with sequence numbers across the N channels (processes) where the sequence numbers are long enough to guarantee in-sequence delivery of packets (RLC-PDUs) to the RLC layer by the MAC-hs layer, but short enough so as not to significantly increase the signaling load.